Beginners' Guide to Welding, Part 3

Flames and brazing

Last week in Part 2 of this series we covered the basics of oxy acetylene
gear, including setting the gear up and checking that there are no leaks. Now
it’s time to light the flame and do some brazing, the easiest of the different
oxy welding techniques.

Before lighting the flame you should be wearing welding gloves and welding
goggles, the latter with the tinted lenses flipped up. You should also have
available a flint lighter designed for lighting oxy acetylene flames.

Open the acetylene blowpipe valve a little: in a quiet workplace you should
just be able to hear the gas flowing. Hold the nozzle pointing away from you and
light the acetylene with the flint lighter. When the gas has ignited, open the
acetylene blowpipe valve further until black smoke and soot cease being
produced. Next, flip down the tinted lenses of the goggles and slowly open the
oxygen valve. The colour and shape of the flame will immediately alter. (If the
flame goes out, you’ve probably opened the oxygen valve too quickly and too far.
Turn off the oxygen, turn down the acetylene and start the process again.)

Types of Flame

When you adjust the ratio of oxygen to acetylene you’ll soon see that three
distinctively different flames can be produced.

A carburising (or reducing) flame occurs when there is excess acetylene. In
appearance the carburising flame has three distinctive parts – the outer flame
envelope and two inner cones, where the innermost flame cone is surrounded by a
luminous feather.

Increasing the amount of oxygen will cause one of the inner cones to
disappear. This flame is said to be a neutral flame. The remaining inner cone is
long and sharply defined.

A flame with an excess of oxygen (an oxidising flame) has a shortened
innermost cone.

In most work a neutral flame is used. However, an oxidising flame can be used
on brass alloys, as the loss of zinc is reduced. A carburising flame is used on
steels being hard-faced as the carbon in the excess acetylene is absorbed into
the surface of the steel. Always check during use that the flame remains as you
have set it.

Brazing

Brazing is the easiest of the metal joining techniques achievable with oxy
acetylene gear. It can be used to join steel, copper, brass and some types of
aluminium. Additionally, dissimilar metals can be joined – for example, copper
tube to steel plate.

Brazing Rods

In this type of welding, the brazing rod melts and becomes the glue that
sticks the surfaces together. Therefore, the rod must be matched to the
application – there’s no such thing as a universal rod that will work with all
metals. Brazing rods vary in three characteristics:

1. Thickness

2. Material

3. Flux-coated or bare

Let’s look at them in turn.

As with electric arc welding, the diameter of the road should be
proportionally matched to the thickness of the material being welded. That is,
the thicker the material, the thicker the rod. If the rod is too thick for the
application, it will take too long to melt and as a result, the materials being
welded may melt rather than just getting dull red. A rod that is too thin will
melt off before the metals being welded are hot enough and so the braze won’t
‘take’.

The material from which the brazing rod is made depends on the application –
that is, the materials being brazed together, the required strength and
appearance. The following table shows the different brazing rods available from
one manufacturer. As can be seen, there are rods to suit different base metals,
different working temperatures and giving different strengths.

BrazeTec Brazing alloy for brazing of

BrazeTec

brazing alloy

Working

temperature in °C

BrazeTec

Flux

Tensile strength of brazed joint (MPa)

Any steels

Copper /

Copper alloys

Nickel /

Nickel alloys

BrazeTec 5600

BrazeTec 4576

BrazeTec 4076

BrazeTec 3476

BrazeTec 4404

BrazeTec 3076

650

670

690

710

730

740

BrazeTec h

350

350

350

360

400

360

Stainless steels

BrazeTec 6009

720

BrazeTec Special h

400

Copper to Copper

BrazeTec Silfos 15

BrazeTec Silfos 5

BrazeTec Silfos 2

BrazeTec Silfos 94

700

710

740

760

n/a

250

BrazeTec brazing alloy for brazing of

BrazeTec

brazing alloy

Working

temperature in °C

BrazeTec

Flux

Shear strength of the brazed joint (MPa)

Tungsten carbide to steel

BrazeTec 4900

BrazeTec 49/Cu

BrazeTec 49/Cuplus

690

690

690

BrazeTec spezial h

BrazeTec spezial h

BrazeTec spezial h

250 – 300

150 – 300

200 – 300

Bare rods require the addition of a flux. The flux, which cleans the base
materials of surface oxides, is applied to the work either directly by means of
a brush or by heating the rod and then dipping it in the flux, so causing the
flex to adhere to the rod. Flux-coated rods come with the flux already on the
rod.

Step by Step - courtesy www.BrazeTec.com

Step 1 – Cleaning

Oxide layers and foreign matter such as rust and scales must be removed from
the brazing joint either mechanically or chemically before brazing. In the case
of sensitive workpieces, thick layers of grease or oil can be wiped off or
removed with solvents (e.g. acetone). Polished workshop pieces do not require
any cleaning. Any oxide remaining on the workpiece after precleaning will be
dissolved by the flux.

Step 2 - Applying flux

The flux paste is applied to the cold workpiece using a brush. Most fluxes
are slightly corrosive and skin contact, particularly with wounds, should be
avoided.

Step 3 - Fixing the workpieces

The pieces to be joined must be fixed in the correct position until the
brazing alloy sets. A narrow brazing gap of between 0.05 mm (0.002") and 0.2 mm
(0.008") is to be set if possible.

Step 4 - Heating the brazing joint evenly

The brazing gap must be heated evenly to working temperature so that the
brazing alloy can fill the gap. The brazing alloy selected should reach working
temperature within 3 minutes at most. Overheating will damage the braze and the
workpiece.

Step 5 - Placing the brazing alloy on the brazing gap

The brazing alloy can be placed on the brazing gap when the flux has melted
to an even glass flow and the working temperature has been reached. The brazing
alloy fills the narrow brazing gap and rises upwards against gravitational
force.

Step 6 - Cooling

When the brazing alloy has filled the brazing gap, the workpiece must be left
to cool until the brazing alloy returns to its solid state. The workpiece can
then be removed from the clamp and then rinsed in water.

Step 7 - Removing flux residue

Residual flux must be removed after brazing to prevent corrosion. Where
possible, use water or a brush to remove any flux residue.

Technique

As with all welding, best results come from practice.

Use a neutral to slightly oxidising flame. Hold the blowpipe so that the
inner cone of the flame is just above the workpiece and heat the two surfaces
until they are a dull red (in the case of steel). Introduce the flux-coated rod
(or having previously applied flux to the join) and the brazing rod should melt
on application to the metals. The brazing material should flow into and along
the join, following the heat.

Where the gap is very small, just a tiny amount of rod is needed. In this
situation, apply the rod and then withdraw it, then using the flame to heat the
metal ahead of the braze and so make it flow forward into the joint. Where the
gap is larger, or a fillet is to be built up, make sure that the brazing
material has first taken to the surfaces before applying more rod and building
the fillet.

If you get the join too hot, the brazing material will sizzle and spark; too
cold and the braze will sit in blobs and not flow into and along the surface.

Control is obtained by removing and applying the flame as necessary to
maintain the correct heat, and removing and applying the rod as required to add
the correct amount of filler material.

Joins with very small gap clearances will not need cleaning up but where
there was a larger gap or a fillet has been created, mechanical sanding and
grinding can be used to shape the join.

Conclusion

The benefits of brazing include reduced heat when compared with welding, so
resulting in less distortion of the workpiece. The fact that the parent material
is not melted means it’s far easier to weld very thin gauges, and – where
tight-fitting joins are being brazed – the result is very neat without further
work being needed. Brazing is also very easy to do, even by a beginner. The
downsides are that the ultimate strength is usually lower than achieved by
fusion, arc, MIG or TIG welding (and that’s especially the case at very high
temperatures), and the brazing process is usually slower than electric welding.
Finally, some brazing rods are quite expensive.

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